1. Field of the Invention
The present invention relates to a vacuum interrupter, more particularly to a vacuum interrupter including an electrode of a magnetically arc-rotating type (hereinafter, the interrupter is referred to as a vacuum interrupter of the magnetically arc-rotating type).
2. Description of the Prior Art
Recently, it has been required to provide a vacuum interrupter of the same size or less as a conventional interrupter and which enhances large current interrupting capability and dielectric strength to cope with increasing of an electric power supply network.
A vacuum interrupter of the magnetically arc-rotating type includes a vacuum envelope and a pair of separable electrodes within the envelope. At least one electrode of the pair is disc-shaped and has a plurality of slots for arc rotation therein, a lead rod which is secured by brazing to the central portion of the backsurface of the electrode and electrically connected to an electric power circuit at an outside of the envelope, and a contact-making portion provided at the central portion of the surface of the electrode. The one electrode outwardly radially and circumferentially drives an arc which is established between the electrodes, by an interaction between the arc and a magnetic field which is produced by arc current flowing radially and outwardly from the contact-making portion to the one electrode during a separation of the electrodes, and by virtue of the slots. Consequently, the one electrode prevents an excessive local heating and melting of the electrodes, thus enhancing the large current interrupting capability and dielectric strength of the vacuum interrupter.
The structure of the electrode and the characteristics of electrode material contribute to a large extent to increasing both the large current interrupting capability and the dielectric strength of the interrupter.
Generally, the electrode itself is required to consistently satisfy the following requirements:
(i) increasing large-current interrupting capability, PA1 (ii) increasing dielectric strength, PA1 (iii) increasing small leading-current interrupting capability and small lagging-current interrupting capability, PA1 (iv) reducing the amount of current chopping, PA1 (v) posessing low electrical resistance, PA1 (vi) possessing excellent anti-welding capability, and PA1 (vii) possessing excellent anti-erosional capability.
However, an electrode consistently satisfying all the above requirements, in the present state of the art, has not been provided.
For example, as an electrode of a conventional vacuum interrupter, there is known an electrode of which a magnetically arc-rotating portion is made of copper and of which a contact-making portion is made of Cu-Bi alloy such as Cu-0.5Bi alloy that consists of copper and 0.5% bismuth by weight added as shown in U.S. Pat. No. 3,246,979; Another example is known of an electrode of which a magnetically arc-rotating portion is made of copper and of which a contact-making portion is made of Cu-W alloy such as 20Cu-80W alloy that consists of 20% copper by weight and 80% tungsten by weight as shown in U.S. Pat. No. 3,811,939.
According to the above electrodes, small mechanical strength, i.e., about 196.1 MPa (20 kgf/mm.sup.2) in tensile strength, of copper causes a magnetically arc-rotating portion to be shaped thick and heavy so that the magnetically arc-rotating portion might prevent a deformation thereof due to a mechanical impact and an electromagnetic force from large current which is applied to the pair of electrodes when a vacuum interrupter is closed and opened. However, it increases a size of the vacuum interrupter.
Additionally, according to the magnetically arc-rotating portion which is thickened and heavy, portions defined by a plurality of slots (hereinafter, referred to as fingers) cannot be lengthened due to the mechanical performance in order to enhance a magnetically arc-rotating force so that the vacuum interrupter difficulty enhances the large-current interrupting capability.
Additionally, the fingers may be eroded by excessive melting and evaporation thereof due to a large current arc because copper and Cu-0.5Bi alloy are soft, and have a vapor pressure considerably higher than that of tungsten and a melting point considerably lower than that of tungsten.